Pierce-type electron gun with combined modulating and beam-forming electrode array



Apnl 9, 1968 AMBQSS ET AL I 3,377,502

PIERCE'TYPE ELECTRON GUN W'ITH' CQMBINED MODULATING AND BEAM-FORMINGELECTRODE ARRAY Filed April 11, 1966 Kurt Amboss, William E. Kramer,

INVENTORS. v

ATTORNEY.

United States Patent PIERCE-TYPE ELECTRON GUN WITH COMBINED MODULATINGAND BEAM-FORMING ELEC- TRODE ARRAY This invention relates to electronbeam generation and modulation, and more particularly relates to amodulating electron gun of the Pierce-type in which the beam modulationis accomplished by means of a unique beam-forming electrode array.

Certain electron beam tubes such as klystrons and traveling wave tubesrequire high density electron beams which are well collimated over adistance many times larger than the transverse dimensions of the beam.Such beams maybe generated by means of electron guns of the Pierce-type,details of which are described in an article by J. R. Pierce,Rectilinear Electron Flow in Beams, Journal of Applied Physics, vol. 11(1940), pages 548 554. Essentially, the Pierce-type gun comprises acathode having a concavely shaped electron-emissive surface, an anodespaced from the cathode along the electron beam pat-h, and abeam-forming electrode longitudinally disposed between the cathode andthe anode. The beamforming electrode has a surface extending outwardlyfrom just beyond the cathode perimeter at angle of essentially 67 /2with respect to the normal to the electron-emissive surface atthecathode perimeter.

It is often desired to operate klystrons and traveling Wave tubes on apulsed basis, and for such operation it is necessary to incorporate amodulating means into the Pierce-type electron gun without impairing thedesired density and collimation characteristics of the generated beam.In one such modulating arrangement the relative potential between theanode and the cathode is Varied on a pulsed basis. However, since therequired change in potential between a level at which the beam is cutoil and a typical operating potential when the beam is present is in thevicinity of 5000 volts, such a modulating arrangement requires thegeneration of pulses having magnitudes of around 5000 volts. Elaborateand expensive power supplies must therefore be employed, especially whenpulse durations as short as 2-20 microseconds are desired.

In another beam modulating scheme which has been employed in aPierce-type gun, the modulating potential .is applied to a gridinterposed between the anode and the beam-forming electrode. Althoughthe required change in grid voltage between the desired opera-tingpotential and the beam cut-ofl? level is about two orders of magnitudeless than that required for anode-cathode modulation, a positivelybiased grid not only causes defocusing of the beam, but it also absorbsa substantial number of the beam electrons.

Attempts have been made to modulate the electron beam of a Pierce-typegun by varying the potential of the beam-forming electrode relative tothat of the cathode. Such attempts have not been efiective, however,because during operation of the gun, .on account of the close proximitybetween the cathode and the beam-forming electrode and the hightemperatures present in the vicinity of these electrodes, some of theelectron-emissive material from the cathode deposits upon the adjacentside of the beam-forming electrode. Consequently, when the beam-formingelectrode is made negative with respect to the cathode in order to cutoff the beam, the resultant electric field between the beam-formingelectrode and the cathode causes electrons to be emitted from't-hebeamforming electrode toward the cathode. Thus, an undesired electroncurrent is generated, resulting in higher power dissipation and loweroperating efficiency.

Accordingly, it is an object of the present invention to provide aPierce-type electron gun incorporating means for modulating thegenerated electron beam, and which electron gun has relatively low powerrequirements and high operating efiiciency.

It is a further object of the present invention to provide a modulatingPierce-type electron gun in which the beam defocusing and electronabsorbing effects of grid modulation are eliminated, while at the sametime the change in modulating voltage required for a beam variationbetween typical operating beam currents and beam cut-oil is an order ofmagnitude less than the modulating voltage change which would berequired if the modulating voltage were applied between the cathode andthe anode.

It is a still further object of the present invention to provide aPierce-type electron gun which is capable of efiiciently genera-tinghigh density, well col-limated electron beam pulses of durations of theorder of 220 microseconds.

In accordance with the foregoing objects, an electron gun according tothe present invention includes a cathode having a concaveelectron-emissive surface facing a predetermined longitudinal directionand an anode spaced from the cathode along the predetermined direction.The anode has an electron beam aperture which is aligned with at least aportion of the electron-emissive surface. An electron beam-formingelectrode array is spaced from the cathode at a location radiallyoutwardly of the cath; ode and longitudinally substantially between thecathode and the anode to establish a potential profile in the regionbetween the cathode and the anode causing electrons emitted from theelectronemissive surface to converge radially inwardly asthey travellongitudinally toward the anode. The beam-forming electrode arrayincludes a pair of spaced electrode segments defining a gap-containingfrusto-conical surface extending outwardly from the electron-emissivesurface at an angle of essentially 67 with respect to the normal to theelectron-emissive surface at the perimeter thereof. The beam-formingelectrode segment nearer the'cathode is maintained at the sameelectrical potential as the cathode, while a modulating voltage isapplied to the beam-forming electrode segment more remote from thcathode.

Additional objects, advantages and characteristic features of thepresent invention will become readily apparent from the followingdetailed description .of a preferred embodiment of the invention whenconsidered in conjunction with the accompanying drawing in which thesole figure is a schematic longitudinal sectional view illustratng anelectron gun in accordance with the principles of the invention.

Referring with greater particularity to the figure, an electron gunaccording to the invention may be seen to include a tubular cathode 10having a concave end surface 12 which is provided with a coating ofelectron-emissive material such as barium oxide, for example. Thecathode 10 is heated in a conventional manner by means of a filament 14energized from a source of potential 16. An

anode 18 having a cylindrical aperture 20 through which the generatedelectron beam passes is coaxially mounted with respect to the cathode apreselected distance away from the electron-emissive surface 12. Thecrosssectional area of the aperture is less than the area of theelectron-emissive surface 12. The anode 18 is maintained at a suitablepotential, such as +5000 volts, with respect to the cathode 10 by meansof a source of potential 22.

Disposed about the electron beam path longitudinally substantiallybetween the cathode 10 and the anode 18 is a beam-forming, or focusing,electrode array consisting of a first beam-forming electrode 24 and asecond beamforming electrode 26 which also functions as a modulatingelectrode, the electrodes 24 and 26 being separated by a preselectedspacing. The first beam-forming electrode 24 is of an annularconfiguration and is coaxially disposed about the cathode 10 in a mannerleaving an annular space between the outer lateral surface of thecathode 10 and the inner lateral surface of the electrode 24. A portion28 of the beam-forming electrode 24 extends longitudinally beyond theelectron-emissive surface 12 in the direction of the anode 18. Theextending portion 28 of the beam-forming electrode 24 is provided with afrusto-conical aperture defined by a surface 30 which, in accordancewith well-known design criterial for Piercetype guns, is disposed at anangle of 67 /2" with respect to the normal to the emissive surface 12 atthe perimeter of the surface 12. The first beam-forming electrode 24 ismaintained at the same electrical potential as the cathode 10 by meansof a lead 32 connected between the cathode 10 and the electrode 24.

The second beam-forming electrode 26 is also of an annular configurationand is coaxially disposed with respect to the cathode 10 longitudinallysubstantially between the first beam-forming electrode 24 and the anode18. The inner lateral surface of the beam-forming electrode 26 defines afirst portion 34 aligned with the frustoconical surface 30 in .a mannerforming what would be a continuous overall frusto-conical surface exceptfor a gap 36 between the electrodes 24 and 26. The width of the gap 36,i.e. the spacing between the electrodes 24 and 26, must be at leastequal to a distance which ensures that voltage breakdown will not occurbetween the electrodes 24 and 26. An electrode spacing in inches equalto .003 times the maximum potential difference in kilovolts between theelectrodes is usually sufficient to prevent voltage breakdown. The innerlateral surface of the beam-forming electrode 26 which is longitudinallymore remote from the cathode 12 than the surface 34 defines a radiallyinwardly projecting annular ridge 38 which causes equipotential lines inthe cathode-anode region to lie more nearly parallel to the cathodesurface 12 over a larger portion of the cathode-anode region than in theabsence of the ridge 38, thereby providing a more homogenous electronbeam.

The second beam-forming electrode 26 may be biased for quiescentoperation in which the electron beam is cut off by means of a voltagesource 40 providing a potential relative to the cathode 10 of 500 volts,for example. A modulating voltage source 42 is connected in series withthe biasing source 40 in order to furnish voltage pulses during thoseintervals of time when it is desired to generate the electron beam. Thepulses provided by the source 42 may be of a magnitude of +500 volts andmay have a duration of 2-20 microseconds, for example.

In the operation of the electron gun of the present invention, underquiescent conditions the modulating pulse source 52 provides an outputof zero volts so that the potential source 40 biases the beam-formingelectrode 26 at a potential sufficiently negative with respect to thecathode 10 to prevent the flow of electrons between the cathode 10 andthe anode 18. When it is desired to generate an electron beam pulse, themodulating source 42 is triggered to provide an output pulse whichessentially cancels the negative bias from the source 40 so as to bringthe beam-forming electrode 26 to essentially cathode potential. Anelectron beam is then able to flow from cathode 10 toward the anode 18,with the potential profile in the anode'cathode region being such as tocause electrons emitted from the surface 12 to converge radiallyinwardly as they travel longitudinally toward the anode 18 in accordancewith well-known Pierce conditions.

Although some electron-emissive material from the cathode surface 12 maydeposit upon adjacent surfaces of the beam-forming electrode 24 duringoperation of the electron gun, since the electrode 24 is alwaysmaintained at the same potential as the cathode, current flow from thefocusing electrode 24 back to the cathode 10 is substantially prevented,thereby eliminating undesired current and improving operatingefiiciency. Moreover, since the focusing electrode 26 affords thedesired beam modulation, the need for a grid is eliminated, therebyavoiding geam defocusing and electron absorbing effects inherent in gridoperation. At the same time, the magnitude of the modulating pulsesrequired for operation of the electron gun of the present invention isan order of magnitude less than that which would be required if themodulating voltage were applied between the cathode and the anode.

Although the present invention has been shown and described withreference to a particular embodiment, nevertheless various changes andmodifications obvious to a person skilled in the art to which theinvention pertains are deemed to lie within the spirit, scope andcontemplation of the invention as set forth in the appended claims.

What is claimed is:

1. An electron gun comprising: a cathode having a concaveelectron-emissive surface facing a predetermined longitudinal direction,an anode spaced from said cathode along said predetermined direction andhaving an electron beam aperture aligned with at least a portion of saidelectron-emissive surface, electron beam focusing means spaced from saidcathode at a location radially outwardly of said cathode andlongitudinally substantially between said cathode and said anode forestablishing a potential profile in the region between said cathode andsaid anode causing electrons emitted from said electr-on-emissivesurface to converge radially inwardly as they travel longitudinallytoward said anode, said focusing means including a pair of spacedelectrode segments defining a gap-containing frusto-conical surfaceextending outwardly from said electron-emissive surface at an angle ofessentially 67 /2 with respect to the normal to said electron-emissivesurface at the perimeter thereof, means for maintaining the focusingelectrode segment nearer said cathode at the same electrical potentialas said cathode, and means for applying a modulating voltage to thefocusing electrode segment more remote from said cathode.

2. An electron gun comprising: a cathode having a concaveelectron-emissive surface facing a predetermined longitudinal direction,an anode spaced from said cathode along said predetermined direction andhaving an electron beam aperture aligned with at least a portion of saidelectron-emissive surface, a first annular beam-forming electrode spacedfrom said cathode at a location radially outwardly of said cathode, saidfirst beam-forming electrode defining and inner frusto-conical surfacedisposed adjacent said electron-emissive surface longitudinally betweensaid electron-emissive surface and said anode, said frusto-conicalsurface extending outwardly from said electron-emissive surface at anangle of essentially 67 /2 with respect to the normal to saidelectronemissive surface at the perimeter thereof, a second annularbeam-forming electrode spaced from said first beamforming electrode andsubstantially disposed radially outwardly of said first beam-formingelectrode longitudinally between said first beam-forming electrode andsaid anode, said second beam-forming electrode defining an innerfrusto-conical surface aligned with said frusto-conical surface of saidfirst beam-forming electrode, means for maintaining said firstbeam-forming electrode and said cathode at the same electricalpotential, and means for applying to said second beam-forming electrodean electrical potential variable between a first level of potentialsufficient to prevent the flow of electrons between saidelectron-emissive surface and said anode and a second level of potentialessentially equal to the potential of said cathode.

3. An electron gun according to claim 2 wherein the spacing in inchesbetween said first and second beamforming electrodes is not less than.003 times the differ- 6 ence inkilovolts between said first and secondlevels of potential.

4. An electron gun according to claim 2 wherein said second beam-formingelectrode further defines a radially inwardly projecting annular ridgein a portion longitudinally more remote from said cathode than theportion of said second beam-forming electrode which defines its saidfrusto-conical surface.

No references cited.

RODNEY D. BENNETT, Primary Examiner. J. G. BAXTER, Assistant Examiner.

1. AN ELECTRON GUN COMPRISING: A CATHODE HAVING A CONCAVEELECTRON-EMISSIVE SURFACE FACING A PREDETERMINED LONGITUDINAL DIRECTION,AN ANODE SPACED FROM SAID CATHODE ALONG SAID PREDETERMINED DIRECTION ANDHAVING AN ELECTRON BEAM APERTURE ALIGNED WITH AT LEAST A PORTION OF SAIDELECTRON-EMISSIVE SURFACE, ELECTRON BEAM FOCUSING MEANS SPACED FROM SAIDCATHODE AT A LOCATION RADIALLY OUTWARDLY OF SAID CATHODE ANDLONGITUDINALLY SUBSTANTIALLY BETWEEN SAID CATHODE AND SAID ANODE FORESTABLISHING A POTENTIAL PROFILE IN THE REGION BETWEEN SAID CATHODE ANDSAID ANODE CAUSING ELECTRONS EMITTED FROM SAID ELECTRON-EMISSIVE SURFACETO CONVERGE RADIALLY INWARDLY AS THEY TRAVEL LONGITUDINALLY TOWARD SAIDANODE, SAID FOCUSING MEANS INCLUDING A PAIR OF SPACED ELECTRODE SEGMENTSDEFINING A GAP-CONTAINING FRUSTO-CONICAL SURFACE EXTENDING OUTWARDLYFROM SAID ELECTRON-EMISSIVE SURFACE AT AN